On July 23, I was on a boat. I like boats. I particularly like the kinds of birds you see while on boats. I also particularly like the kinds of birders who like being on boats and who like the kinds of birds you see while on boats (recently mentioned exceptions notwithstanding).

This particular boat was the Condor Express out of Santa Barbara, and the trip was great for diversity, if not for Pterodroma species (no puling from me, however, I saw over 20 year birds over the course of 14 hours). Did you know that “petrel” is probably derived from “St. Peter”? We like to think about the fine points of flight differences between shearwaters and gadfly petrels (and very much enjoy the ensuing heart murmur/attack when a bird has arced up over the waves in a gadfly fashion), but somehow I had not connected the general word petrel with the idea of walking on water. Sort of like how I had not connected Kanga and Roo until about the age of 25: we all have our moments of brilliance, I guess.

The bird of the day (after the more legitimate excitements of a Laysan Albatross, 2 Red-billed Tropicbirds, a Manx Shearwater, etc.) ended up being this stupid tern. It should have been a simple matter, but this first year bird confounded me. My first thought was Common Tern, but then I have more experience with those than with Arctic, and not a depth of knowledge in either, so when it was called as Arctic (and almost as quickly debated), I evaluated again. This left me confused. The bird was obliging, however, and sat as we neared it, and provided for perfectly fine photo opportunities. I thought this should then clear everything up – surely this one could be solved? I have no qualms about “letting it go”, but in this case, both in the field and in the photos, I observed features that could argue for either Common or Arctic. My photos are *not* perfectly fine, for the record, because my exposure was stopped down for some previous shots, and because of many other excuses I’ll leave out. They do point out some of the ID arguments, however, so here they are:

Anyone wish to discuss? Also, more photos from the day are at my BirdspotUS Flickr page here, if you are interested.

Edited Aug 19: I have added two more photos. They’re crap, but one gives a view of the bird in a less foreshortened profile, the other possibly more info as to the wing plumage:

16 Responses to “An Easy ID”

Craig – would you be willing to list out why? One, because I am really interested, and two, because there are some very (as in very) good birders who feel this is a Common. The response has been interestingly split.

Its a first summer mainly distinguished by the fact that the primaries are fresh whereas a similar common tern would have worn primaries. In its sitting position the Arctic has a more domehead rather than the common which is quite flat. They also look cuter and more panda like than the common which can look cross.

In flight, juvenile Common Tern has a prominent dark bar along the leading edge of its inner wing. Overall, its upperwing is well patterned but clean, bright white is missing whereas Juvenile Arctic Tern has a tricolour pattern, ranging from smoky grey along the forewing through a light grey mid-wing to, very importantly, a blinding white trailing edge. Especially, the white trailing edge is at its widest midway along the trailing edge where it widens into a delta shape, echoing the shape of the wing itself.

Awesome Craig – I can’t thank you enough for taking some time to respond. As to the age of this bird, is it at all possible that this could be a 2nd year bird? I am hoping that someone (anyone) with more experience than I will present the arguments for Common, but if no one chimes in, I’ll stand up to the plate… (very American saying, that).

Don’t the legs seem a little long for Arctic ?
The head is not in profile. This is problematic for estimating bill size with respect to body, but that doesn’t make it an “obvious” Arctic to me either.

I’m not sure if Craig is arguing for juvenile or 1 year old, but I think the tail shape (especially the outer retrices) might be an indicator of age here. If I ever make it out of the lab I shall pull field guides later.

An interesting bird…first impression based on general size and structure, leg length and bill size and shape is Common Tern. The pale orbital rings seem very prominent and un-Arctic like, reinforcing the Common Tern look.

However, I do agree with Craig that the freshness and uniformity of the primaries and a narrow white trailing edge are features more in line with Arctic at this time of year. 2nd Cal-yr Commons due to moult timing generally show more worn outer primaries. But, can we conclusively age this as a 2nd-cal yr?

Are there any other photos, since the pictures of the bird on the deck might be misleading. With a little more focus on the photos and moult I can see why the bird has split people.

However, to me, the bird doesn’t look like an Arctic Tern -it doesn’t look compact, bull-necked with a domed head..and the pale orbital rings are probably atypical for Arctic.

I am arguing for 1yr old. The retrices on the tail of an arctic tern tend to be longer than the wings at rest whereas with common the wings protrude beyond its tail retrices as can also be seen in the photo at 10000 birds

There are better shots out there – at least two birders with great photo kit and stellar credentials got shots, but interestingly were also divided as to Arctic or Common. I’ll see if I can track them down.

I have put two more (poor) photos up – one with a better indication of the head and bill in profile, and another blurry flight shot.

Thank you again! What a great learning opportunity; the fact that anyone is willing to comment is heartening.

With the new images posted, they cement my opinion that this is a Common Tern based purely on structure and proportions. I feel the plumage and structure outweigh the moult difference, but I am not experienced enough to discuss why.

Perhaps those that favor Arctic could expound on the shape of the bird and the apparent legginess?

And, though I am far from an expert (I just like to listen to experts), I would go with Common Tern on this bird just based on the length of the legs. Arctic Terns have absurdly short legs and this bird doesn’t.

Hi Catherine and all, I’m confident this is an Arctic Tern. Craig and others have mentioned many of the marks indicating this, but I’d like to emphasize the ones that are least ambiguous.

1. Head pattern. The combination of the black mask extending forward to encompass the eye from above, below, and in front and the white forehead extending far rearward on the crown is diagnostic for Arctic vs. Common. The latter’s mask constricts behind the eye, so that the small black patch in front of the eye appears isolated from the black behind the eye. The whitish arcs immediately above and below the eye are fine for yearling Arctic; a similarly plumaged Common would never show so much dusky below the eye.

2. The length of the “hand” relative the rest of the wing–and the rest of the body. On both the flight view and the view of the perched bird in profile, the hands are strikingly long, slender, and pointed relative to other landmarks (e.g., total body length, inner arm, etc.). This is difficult to describe but quite obvious with practice.

3. The black trailing edge to the underside of the hand is longer and thinner than it would appear on a Common Tern. This feature can be very difficult to see in the field, but is quite easy to assess on these photos.

The leg length doesn’t concern me for Arctic. This feature can be surprisingly difficult to assess. Ideally one would like to see the whole the tarsometatarsus clearly (between the ankle joint and where the toes come off). If one can’t see both ends, the long legs of a Common can look short and the extended legs (including parts above the ankle) of an Arctic can look long. This bird’s bill and (domed) head shape look spot-on for Arctic.

The following two albums show lots of photos of similarly plumaged Arctic and Common Terns:

Regarding the tail feather length and pattern, yearling Arctic shows longer outer tail feathers than does yearling or juv Common; this bird’s streamers are quite long, consistent with Arctic. In both species, the outer webs of the outer two tail feathers are blackish. This can be difficult to see in life–and even in photos, because the pigmented webbing is thin and its appearance varies greatly with lighting.

The whole process has been elucidating – there are things that for me are more obvious as an artist than as a birder. I understand exactly what you mean by the length of the hand relative to the body. I just had another experience of this sort when in Kansas a few weeks ago: in watching thousands of Franklin’s Gulls swirling over Cheyenne Bottoms I was bowled over by how different they seemed to Laughing Gulls (more experience with the latter; I had only seen 3-4 Franklin’s previously). My first thoughts were “wow dumpy stiff alcid-like gulls” and from there I looked carefully at the wing to body proportions and how that might affect their flight. Fascinating, even when I am only hypothesizing.